(HealthDay News) -- Kids at the head of the class not only have better college and job prospects, they may also stave off Alzheimer's disease, two new studies suggest.

People with the best school grades and most complex jobs later on -- as managers, teachers or executives, for example -- have roughly a 40 to 60 percent reduced risk of developing dementia, according to two teams of Swedish researchers.

But, both Swedish studies only found an association between school grades, later jobs and risks of dementia, and not a proven cause-and-effect link.

"We are just starting to figure out that there is this lifetime risk for developing Alzheimer's disease," said Dean Hartley, director of science initiatives at the Alzheimer's Association, which played no part in either study.

Part of reducing the risk for dementia is to build up what experts call "cognitive reserve," he said.

Cognitive reserve is established by building many links -- called synapses -- between brain cells. It seems that the more you use your brain, the more connections are created between brain cells, Hartley said.

A complex job that requires a lot of brain work can preserve those cell links and build even more, Hartley said.

In turn, "having more connections in your brain allows you to maintain more of your thinking and more memory abilities," he said.

In one of the Swedish studies, a research team led by Serhiy Dekhtyar, from the clinical neuroscience department at the Karolinska Institute in Stockholm, followed more than 7,500 people 65 and older who took part in the Uppsala Birth Cohort Study for more than 20 years. Data on participants' childhood mental ability was also collected.

The researchers found that 950 people developed dementia, and risk for mental decline increased 21 percent among those whose grades were in the bottom 20 percent of the overall Swedish population.

But risk for dementia decreased 23 percent among those who had jobs that required complex thinking and work with data and numbers -- the managers, teachers and executives, Hartley said.

People who had both high grades in school and a complex job later in life had the highest reductions in the risk for dementia -- 39 percent, the researchers found.

"Our findings highlight the importance of early life cognitive performance for the late-life risk of dementia," Dekhtyar said in a news release. "It appears that baseline cognitive ability -- even at age 10 -- may provide the foundation for successful cognitive aging much later in life."

In the other study, a team also from the Karolinska Institute found that dementia risk was increased more than 50 percent in people over 75 who had the lowest 20 percent of school grades, even if they had more formal education or a job requiring complex thinking.

High school graduates had a 28 percent lower risk of dementia compared to those with only an elementary school education, the researchers say.

In addition, women with complex jobs requiring negotiating, instructing and supervising people had a 60 percent lower risk of developing dementia compared with those who had less complex jobs, the researchers found.

The researchers' conclusions were based on data on 440 men and women 75 and older who were followed for nine years. During that time, 163 study participants developed dementia.

Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at Cohen Children's Medical Center of New York in New Hyde Park, said the studies' findings offered little practical application for children.

"It is unrealistic to think that a 9-year-old will apply himself more in school just so that he will have a more fulfilling quality of life 50 or 60 years later," he said.

Also, "a cynic might argue that there will always be a bottom 25 percent in terms of test performance and grades," Adesman added.

However, enough other sound reasons exist why students should apply themselves in school, he said.

Dr. Luca Giliberto, an Alzheimer's investigator at the Feinstein Institute for Medical Research in Manhasset, N.Y., said mental activity is good preventive medicine. "Being mentally and physically active gives you an advantage in reducing the risk of Alzheimer's and will help you build that cognitive reserve," Giliberto said.

The results of these studies were scheduled for a recent presentation at the Alzheimer's Association International Conference, in Washington, D.C. Data and conclusions presented at meetings are usually considered preliminary until published in a peer-reviewed medical journal.